Many electronic devices, regardless of application, include system timing clocks and other high-speed components that can produce electromagnetic radiation. Electromagnetic radiation emitted from an electronic device can interfere with other devices in the vicinity of the electronic device. For example, electromagnetic radiation in the radio frequency (RF) spectrum is referred to as RF radiation and can result in RF interference (RFI) when it interferes with electronic devices located in the vicinity of the emitting device. The radio frequency radiation emitted from an electronic device can be a complex mixture of high and low frequencies, their associated harmonics, as well as broadband noise.
To control and regulate the amount of radio frequency radiation emitted by an electronic device, regulatory agencies and certification authorities have developed limits of allowable emissions under which all electronic devices must remain. For example, the Federal Communication Commission (FCC) determines maximum radio frequency radiation limits for electronic devices sold in the United States. Manufacturers of electronic devices are required to make their products comply with such regulations. Unfortunately, as clock frequencies and operating frequencies of electronic devices continue to increase, frequently so does the amount of radio frequency radiation emitted by these devices. When a device is compliance tested it is required that all connectors and cables that are provided with the device be connected to the device. At high frequencies, these connectors and cables tend to behave as transmission lines or antennas. Thus, connectors and cables facilitate the emission of radio frequency radiation. The electrical behavior of cables is modeled by distributed inductance and capacitance.
Electronic signals are frequently referred to as either “common-mode” or “differential-mode.” Common-mode can be defined as the instantaneous algebraic average of two signals applied to a balanced circuit, where both signals are referred to a common reference. Differential-mode can be defined as the instantaneous algebraic difference between two signals applied to a balanced circuit, where both signals are referred to a common reference. The signals may be either voltages or currents.
FIG. 1 is a schematic diagram 1 illustrating differential-mode and common-mode signals. The diagram 1 includes an electronic device 2 coupled to another electronic device 3 via an interconnect cable, illustrated using a pair of conductors 4 and 5. For purposes of illustration, conductor 4 carries a signal and conductor 5 provides a return path for the signal. The signal carried in the conductors 4 and 5 is considered differential mode if the net sum of the currents in conductors 4 and 5 is equal to zero. Unfortunately,an interconnect system as shown in FIG. 1 frequently includes a parasitic capacitance associated with each electronic device. The parasitic capacitance associated with electronic device 2 is illustrated using capacitive element 6 and the parasitic capacitance associated with electronic device 3 is illustrated using capacitive element 7. The capacitive elements 6 and 7 couple the electronic devices 2 and 3, respectively, to ground. The parasitic capacitances 6 and 7 provide an alternative path between electronic devices 2 and 3. This alternative path allows undesirable current to flow between the electronic devices 2 and 3. The undesirable current is illustrated using arrows 8 and is referred to as a common-mode current. The common-mode current can give rise to radio frequency radiation. The common-mode current may be attenuated by attaching what is referred to as a “common-mode filter” to the cable carrying the conductors 4 and 5. A common-mode filter is typically a toroidal shaped ferrite device. Such a device provides lossy attenuation of the common-mode current, while not inductively loading the differential-mode signal in the cable. The ability of such a ferrite device to attenuate the unwanted common-mode current (and therefore reduce unwanted radio frequency radiation) while not degrading the differential-mode signal is largely a function of the material from which the ferrite device is constructed and the geometry of the device.
The effectiveness of a ferrite-based device at suppressing radio frequency radiation is limited by the net current passing through the ferrite device. Large amplitude, low-frequency signals may easily bias, or saturate, a ferrite device so that it becomes ineffective at other frequencies. This situation becomes more problematic when attempting to suppress radio frequency radiation on a single conductor, where the net current on the conductor is non-zero.
FIGS. 2A and 2B are graphical illustrations depicting the limitations of a conventional ferrite-based device at suppressing radio frequency emission occurring at more than one frequency. In FIG. 2A a current at a frequency of f0 is represented on the conductor 4 using time-varying trace 15. The trace 15 includes a node 16 at which the current at frequency f0 is zero and anti-nodes 14 at which the current amplitude is at a maximum, represented by the arrows 12. The current amplitude of the trace 15 is represented as a function of position along the conductor 4 at a frequency f0. In FIG. 2B, a conventional ferrite device 18 is located on the conductor 4 at an anti-node 14, which coincides with the maximum current amplitude of the frequency f0 carried in the conductor 4. Unfortunately, this placement of the ferrite device 18 is sub-optimal for suppressing radio frequency emissions at a frequency of f1, where f1 is greater than f0 because the ferrite device 18 tends to saturate with the current generated by the signal at frequency f0. This causes the ferrite device 18 to become ineffective at suppressing radio frequency emissions at a frequency of f1, where f1 is greater than f0.
FIGS. 3A and 3B are block diagrams illustrating the saturation of a ferrite device. FIG. 3A illustrates a ferrite device 18 in an unsaturated condition. The ferrite device 18 is typically fabricated of a compound including iron oxide and includes a plurality of micro-domains, an exemplary one of which is illustrated using reference numeral 22. In an unsaturated condition (i.e., in the absence of an external magnetic field created by current in the conductor4), the micro-domains are randomly oriented, as illustrated using arrows 24.
In FIG. 3B, an external field caused by current in the conductor 4 and illustrated using arrow 25 is applied to the ferrite device 18. The external field 25 causes the boundaries in the micro-domains 22 to shift and the micro-domains to substantially align with each other, as illustrated with arrows 24 aligned in generally the same general direction. This alignment of the micro-domains results in an energy transfer from the conductor 4 to the ferrite device 18 at a particular frequency (i.e., at frequency f0). The energy absorbed by the ferrite device 18 at frequency f0 results in reduced radio frequency emission from the conductor 4 at frequency f0. Unfortunately, the energy absorbed by the ferrite device 18 at frequency f0 may saturate the ferrite device 18 and prevent the ferrite device from further absorbing energy from a conductor that also includes a higher frequency current, such as at a frequency f1, where f1 is greater than f0.
Therefore, it would be desirable to have a way of enhancing the radio frequency radiation suppression characteristics of a ferrite device.